Integrated hydrostatic transaxle with controlled traction differential

ABSTRACT

An integrated hydrostatic transaxle including a housing in which a center section is supported. The center section supports a hydraulic pump unit and a hydraulic motor unit having a motor shaft drivingly connected thereto. A differential assembly is drivingly linked to the motor shaft and is used to drive a pair of axle shafts which are supported by the housing. The differential assembly includes a rotatable gear which is maintained in frictional engagement with at least one bearing surfaces for frictionally inhibiting the rotational movement thereof.

This application is a continuation of application Ser. No. 08/698,650,filed Aug. 16, 1996.

BACKGROUND OF THE INVENTION

This invention relates generally to transaxles and, more particularly,relates to an integrated hydrostatic transaxle comprising a controlledtraction differential.

As is known, the use of a standard differential assembly in thetransmission system of a motor vehicle allows the wheels to spin atdifferent speeds. In the case of a vehicle traveling in a straight line,the axle shafts connected to the standard differential assembly willrotate at the same speed. However, when a turn or curve is encountered,the axle shaft nearest the inside of the turn will slow in rotationalspeed while the outer axle shaft will simultaneously increase inrotational speed. As such, the wheels, driven by the axle shafts, areprevented from scuffing the surface across which they travel.

While the standard differential assembly serves an important function inthe operation of a transmission system, the standard differential ofsimple construction has difficulty operating under certain conditions.For example, when a first one of the drive tires is disposed in wet,muddy, or loose soil conditions, or when the first drive tire has beenpartially or completely removed from contact with the ground, thecoefficient of friction under the first drive tire will be substantiallylower than that associated with the second drive tire. This resultingfrictional imbalance will tend to cause the second drive tire to remainstationary while the first drive tire will spin without moving thevehicle.

To solve the problem of loss of traction in larger transaxle systems, avariety of techniques have been developed to reduce the tendency of thetransaxle to differential. For example, U.S. Pat. No. 3,528,323 toKamlukin, issued Sept. 15, 1970, discloses a means for preventing freespinning of one of the driven shafts of a transaxle without interferingwith the normal differential capability of the transaxle. In particular,the '323 patent discloses the use of a coil spring to outwardly forcethe gears of the differential into engagement with the differentialhousing thereby introducing a frictional force into the assembly whichacts to resist relative rotation of the axle shafts. In this manner, thefrictional force limits the free spinning of one axle shaft while theother remains stationary.

While the friction inducing means disclosed in the '323 patent works forits intended purpose to provide larger transaxles with a controlledtraction differential assembly, the use of coil springs to apply thefrictional force often requires special tools and/or procedures for usein applying the forces necessary to set the springs. These additionaltools and/or procedures undesirably results in increased manufacturingcosts. Therefore, there remains a need for a controlled tractiondifferential assembly which is simpler to construct, can be produced ata lower cost, and which can be readily incorporated into a smallertransaxle such as an integrated hydrostatic transaxle.

As a result of these existing needs, it is an object of the presentinvention to provide an integrated hydrostatic transaxle having acontrolled traction differential assembly which will provide thehydrostatic transaxle with the benefits and advantages which haveaccrued to other types of transaxles that use controlled tractiondifferential assemblies.

It is a further object of the present invention to provide a controlledtraction differential assembly which is cost effective and relativelyeasy to manufacture.

It is still a further object of the present invention to provide acontrolled traction cartridge for use in conveniently converting astandard differential assembly into a controlled traction differentialassembly.

It is yet a further object of the present invention to provide acontrolled traction differential assembly in which the breakdown bias,i.e., the amount of torque required to cause the differential tooperate, may be easily varied.

SUMMARY OF THE INVENTION

In accordance with these objects, an integrated hydrostatic transaxle isprovided. Generally, the transaxle comprises a hydrostatic transmissionincluding a center section on which is supported a hydraulic pump unitand a hydraulic motor unit and a motor shaft drivingly connected to thehydraulic motor unit. A differential assembly is drivingly linked to themotor shaft for use in driving a pair of axle shafts. The differentialassembly comprises a pair of gears rotatable with respect to each otherand a friction inducing means for use in frictionally inhibiting themovement of at least one of the pair of gears with respect to the otherof the pair of gears. In this manner the frictional force applied to thegear prevents normal operation of the differential when the drive tiresmounted upon the axle shafts are under conditions of frictionalimbalance.

More specifically, the differential assembly includes a pair of gearsrotatable with respect to each other which are both disposed between aninterior and an exterior bearing surface. The interior and the exteriorsurfaces are compressed against the pair of gears to maintain the gearsin frictional engagement therewith thereby inhibiting the rotationalmovement of the pair of gears with respect to one another. In apreferred embodiment of the invention, the interior bearing surface isprovided by forming at least one shoulder on a cross shaft whichsupports the pair of gears and the exterior bearing surface is providedby a pair of bearing blocks.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawings which set forth anillustrative embodiment and is indicative of the various ways in whichthe principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to thepreferred embodiment shown in the following drawings in which:

FIG. 1 illustrates a sectional side view of an integrated hydrostatictransaxle having a controlled traction differential assembly inaccordance with the present invention;

FIG. 2 illustrates a partial sectional bottom plan view of theintegrated hydrostatic transaxle taken along line II--II in FIG. 1;

FIG. 3 illustrates a sectional side view of the integrated hydrostatictransaxle taken along line III--III in FIG. 1;

FIG. 4 illustrates a close-up view of the controlled tractiondifferential assembly as illustrated in FIG. 1;

FIG. 5 illustrates a close-up view of the controlled tractiondifferential assembly as illustrated in FIG. 2;

FIG. 6 illustrates an isometric view of the controlled tractiondifferential cartridge used in connection with the invention asillustrated in FIGS. 1-5; and

FIG. 7 illustrates a sectional side view of the assembled controlledtraction differential cartridge as illustrated in FIG. 6.

DETAILED DESCRIPTION

While the invention can be used in connection with most types oftransaxles it will be described hereinafter in the context of anintegrated hydrostatic transaxle ("IHT") as the preferred embodimentthereof.

Referring now to the figures, wherein like reference numerals refer tolike elements, there is generally illustrated an IHT 10. While a briefdescription of the general construction and operation of the IHT willfollow, the reader is referred to U.S. Pat. Nos. 5,201,692 and5,314,387, which patents are hereby incorporated by reference in theirentirety, for a more thorough discussion of this subject matter.

As illustrated, the IHT 10 is encased within a housing comprised of afirst housing section 12 and a second housing section 14 joined along asubstantially horizontal split line 16. It is to be understood, however,that the IHT described hereinafter may be disposed within a housingcomprised of any number of housing sections having split lines invarious orientations. Accordingly, the design of the housing illustratedis not meant to be limiting.

The IHT 10 includes a center section 18 having hydraulic porting formedtherein on which are mounted a hydraulic pump unit 20 and a hydraulicmotor unit 22. The particular arrangement of the center section 18,hydraulic pump unit 20, and hydraulic motor unit 22 may be variedaccording to space requirements dictated by the size and configurationof the vehicle with which said IHT is to be employed. Specifically, thehydraulic pump unit 20 generally comprises a pump cylinder block 24having a plurality of piston receiving chambers 26 each of which movablycontains a pump piston 28 and piston spring 30. Similarly, the hydraulicmotor unit generally comprises a motor cylinder block 32 having aplurality of piston receiving chambers 34 each of which movably containsa motor piston 36 and piston spring 38. The hydraulic pump unit 20 ishydraulically connected to the hydraulic motor unit 22 through thehydraulic porting formed in the center section 18.

An input shaft 40, which is driven by the engine of the vehicle (notshown), is drivingly connected to the hydraulic pump unit 20 such thatthe rotation of the input shaft 40 rotates the pump cylinder block 24therewith. The rotation of the pump cylinder 24 causes the pump pistons28 to travel up and down as they travel against a swash plate 42. Theswash plate 42 may be moved to a variety of positions to vary the strokeof pump pistons 28; this varies the volume of hydraulic fluid pumpedinto the hydraulic porting which, in turn, ultimately varies the speedof the hydraulic motor unit 22. Specifically, each motor piston 36 isdriven by the pumped hydraulic fluid against a fixed, angularlyorientated motor thrust bearing 44 such that the action of the motorpistons 36 against the thrust bearing 44 creates a rotational movementof the motor cylinder block 32. Drivingly connected to the motorcylinder block 32 is a motor shaft 46 which accordingly rotatestherewith. A disc brake assembly 47 is also provided and connected tothe motor shaft 46.

In the illustrated embodiment, the motor shaft 46 drives a first gear 48that is drivingly connected to a second reduction gear 50. The reductiongear 50 is drivingly connected to a third, bull gear 52. The bull gear52 imparts the rotational movement translated through the first andsecond gears from the motor shaft 46 to the differential assembly 54. Itis to be understood, however, that the gear configurations describedherein are meant to be illustrative only and that other variations maybe employed without departing from the scope of the invention, e.g., thefirst gear 48 may be arranged to directly drive the bull gear 52 or maybe adapted to drive additional gears for the purpose of providingfurther reduction.

As best seen in FIGS. 4 and 5, the differential assembly 54 generallycomprises a pair of bevel planet gears 56 matingly engaged with the bullgear 52 and, accordingly, rotatable therewith. The bevel planet gears 56are also drivingly connected to a pair of bevel drive gears 58 whichare, in turn, drivingly attached to a pair of oppositely disposed axleshafts 60 which comprise the axle. Specifically, the bevel planet gears56 are engaged with the bull gear 52 through the use of bearing blocks62 which are received in corresponding mating slots formed therewithin.While the preferred embodiment has been illustrated as utilizing twopairs of bevel gears, it will be appreciated by those of ordinary skillin the art that other gearing arrangements may be utilized.

Turning to FIGS. 6 and 7, the bevel planet gears 56 and bearing blocks62 comprise a part of the controlled traction cartridge 64. Morespecifically, the controlled traction cartridge 64 comprises a shaft 66which may be a cross shaft, split shaft, or the like. For ease ofunderstanding the shaft 66 shall be simply referred to herein as crossshaft 66. The cross shaft 66 has an axial opening therethrough and apair of oppositely disposed interior bearing surfaces 68 which may beshoulders formed on the cross shaft 66, retaining rings, or the like.Against the bearing surfaces 68 the bevel planet gears 56 are rotatinglymounted. Additionally, the bearing blocks 62 are also mounted on thecross shaft 66 in communication with the bevel planet gears 56 andprovide an exterior bearing surface 67. Preferably, the ends of thecross shaft 66 are provided with flats 70 which engage a correspondinginterior surface provided to the bearing blocks 62 to prevent therotation of the cross shaft 66 with respect thereto. In furtherembodiments, the flats 70 could be replaced by a spline, serrations, orother like type of rotation resistant features. The prevention of therotation of the cross shaft 66 is preferred since it is seen to maximizethe force required to rotate the bevel gears with respect to oneanother. In addition, each of the bearing blocks 62 is provided with acavity 72 in which is disposed one or more hemispherical spring washers74.

To maintain the arrangement of the components of the controlled tractioncartridge 64 a bolt 76 is utilized. Specifically, the bolt 76 ispositioned generally through the center of the above-describedcomponents and a nut 78 is affixed thereto which nut 78 is trapped inthe cavity 72 in the corresponding bearing block 62. Preferably, thecavity 72 in the corresponding bearing block 62 is adapted to preventthe nut 78 from rotating with respect thereto during assembly whileallowing room for the positioning of the spring washers 74 therewithin.It is further preferred that the nut 78 be tack welded to the bolt 76after the components are fully assembled.

When the controlled traction cartridge 64 is fully assembled, the bolt76 and nut 78 function to compress the spring washers 74 within thecavities 72 of the bearing blocks 62. Thereafter, the action of thespring washers 74 against this compressive force drives the bearingblocks 62 into increased frictional engagement with the bevel planetgears 56 which are, in turn, also driven into further frictionalengagement with the shoulders 68 of the cross shaft 66. This frictionalengagement of the bevel planet gears 56 between the bearing blocks 62and the shoulders 68 functions to inhibit the normal rotational movementof the bevel planet gears 56 which occurs when the transaxledifferentials. Specifically, the bevel planet gears 56 will not rotateor differential until the rotational force or torque imparted thereuponby the bevel drive gears 58 is sufficient to overcome the frictionalforces created by engagement of the bevel planet gears 56 with thebearing blocks 62 and shoulders 68. In this manner, when one of thedrive wheels connected to one of the axle shafts is operating in acondition of reduced friction, the herein described controlleddifferential assembly will maintain both drive wheels in uniformrotation until such time as the torque created by the drive wheels issufficient to overcome the frictional forces applied to the bevel planetgears 56. Furthermore the amount of torque required to rotate the bevelplanet gears 56 of the differential assembly, the breakdown bias, may beeasily adjusted by varying the quantity of spring washers utilized or byusing spring washers with different spring constants. Additionally,other components capable of creating similar forces upon the bevelplanet gears in the arrangement above-described may be utilized such aswave washers, split washers, or the like.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. For example, thearrangement disclosed herein may be modified whereby the frictionalforce is applied to only one of the bevel planet gears. In this mannercontrolled traction may still be achieved. Accordingly, the particulararrangements disclosed are meant to be illustrative only and notlimiting as to the scope of the invention which is to be given the fullbreadth of the appended claims and any equivalent thereof.

What is claimed is:
 1. An integrated hydrostatic transaxle, comprising:ahousing; a hydrostatic transmission mounted within said housingcomprising a hydraulic pump unit and a hydraulic motor unit; a motorshaft drivingly connected to said hydraulic motor unit; a pair of axleshafts supported by said housing; a differential assembly drivinglylinked to said motor shaft for use in driving said pair of axle shaftscomprising at least one gear rotatably mounted on a differential shaft;an interior bearing surface associated with said differential shaftagainst which said gear is forced into frictional engagement; and aspring means cooperable with said gear for use in forcing said gear intofrictional engagement with said interior bearing surface.
 2. Thehydrostatic transaxle as recited in claim 1, wherein said differentialshaft is a single unit, and further comprising a pair of gears mountedon said differential shaft and said pair of gears each comprise a bevelplanet gear.
 3. The hydrostatic transaxle as recited in claim 1, furthercomprising a bearing block disposed between said spring means and saidgear.
 4. An integrated hydrostatic transaxle, comprising:a housing; ahydrostatic transmission mounted within said housing comprising ahydraulic pump unit and a hydraulic motor unit; a motor shaft drivinglyconnected to said hydraulic motor unit; a pair of axle shafts supportedby said housing; and a differential assembly drivingly linked to saidmotor shaft for use in driving said pair of axle shafts comprising apair of gears mounted on a hollow cross shaft and rotatable with respectto each other and a nut and bolt assembly disposed through said crossshaft, said cross shaft comprising a pair of bearing surfaces againstwhich a corresponding one of said pair of gears is forced intofrictional engagement.
 5. The hydrostatic transaxle as recited in claim4, further comprising a spring associated which each of said pair ofgears for forcing each of said pair of gears into frictional engagementwith said corresponding one of said pair of bearing surfaces.
 6. Anintegrated hydrostatic transaxle, comprising:a housing; a hydrostatictransmission mounted within said housing comprising a hydraulic pumpunit and a hydraulic motor unit; a motor shaft drivingly connected tosaid hydraulic motor unit; a pair of axle shafts supported by saidhousing; a differential assembly drivingly linked to said motor shaftfor use in driving said pair of axle shafts comprising a rotatable gearmounted on a differential shaft having a shoulder formed thereon,wherein said gear is disposed between said shoulder and a bearing blockforming an exterior bearing surface; and a spring associated with saidbearing block for use in forcing said bearing block into frictionalengagement with said gear.